# Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore Yb$_2$Ti$_2$O$_7$ in Magnetic Field

## TL;DR

This study investigates the magnetic excitations in the frustrated pyrochlore Yb$_2$Ti$_2$O$_7$ under magnetic fields, revealing quasiparticle breakdown, magnon decay, and a re-evaluated spin Hamiltonian with dominant quantum exchange interactions.

## Contribution

It provides the first detailed inelastic neutron scattering analysis of Yb$_2$Ti$_2$O$_7$ in magnetic fields and proposes a revised spin Hamiltonian emphasizing quantum exchange effects.

## Key findings

- Observation of magnon decay and two-magnon continuum at intermediate fields.
- Smooth connection between low and high field regimes with no sharp phase transition.
- Re-evaluation of the spin Hamiltonian highlighting dominant quantum exchange terms.

## Abstract

The frustrated pyrochlore magnet Yb$_2$Ti$_2$O$_7$ has the remarkable property that it orders magnetically, but has no propagating magnons over wide regions of the Brillouin zone. Here we use inelastic neutron scattering to follow how the spectrum evolves in cubic-axis magnetic fields. At high fields we observe in addition to dispersive magnons also a two-magnon continuum, which grows in intensity upon reducing the field and overlaps with the one-magnon states at intermediate fields leading to strong renormalization of the dispersion relations, and magnon decays. Using heat capacity measurements we find that the low and high field regions are smoothly connected with no sharp phase transition, with the spin gap increasing monotonically in field. Through fits to an extensive data set we re-evaluate the spin Hamiltonian finding dominant quantum exchange terms, which we propose are responsible for the anomalously strong fluctuations and quasiparticle breakdown effects observed at low fields.